Conventionally, as a method for producing a fine pattern in a high-density printed wiring board used for a semiconductor plastic package, there is a method according to a subtractive process in which a through hole and/or a blind via hole is/are made in an ultrathin copper foil having a thickness of 5 μm or less with a mechanical drill or a carbon dioxide gas laser, then electroless copper plating is carried out to form a plating layer having a thickness of 0.5 to 3 μm, a negative type plating resist is formed, a copper plating layer having a thickness of approximately 15 μm is formed, the plating resist is removed and then the electroless copper plating and the ultrathin copper foil are removed by etching (flash etching). Since this method uses a negative type etching film, a negative type dry plate for exposure is placed thereon and exposure is carried out. In this method, it is difficult to form a pattern of 10 μm or less. Furthermore, the above method is insufficient with regard to a pattern location accuracy. On the other hand, there has been widely known a method of producing a pattern of a semiconductor device or a liquid crystal device by the use of a positive type resist. However, there has not been known a method of producing a fine pattern of a printed wiring board by the use of a positive type resist by flash etching.
Further, there is known a method in which a thorough hole and/or a blind via hole is/are made by directly irradiating a copper foil surface with a carbon dioxide gas laser, then copper foil burrs occurring in the hole portion are dissolved and removed, at the same time copper foils as external layers are dissolved and removed by SUEP (Surface Uniform Etching Process) until the thickness of each of the copper foils decreases from 12 μm to 5 μm or less, then a desmearing treatment is carried out, then copper plating is carried out to form a plating layer of approximately 15 μm, and an ultrafine pattern is formed by using a general etching resist. In this method, the upper portion of the pattern becomes narrower than the bottom portion because of the etching so that a cross section becomes trapezoidal or triangular, which causes the occurrence of defectives.
Further, there is a method in which, after plating is carried out by a semi-additive method, an ultrafine pattern is similarly formed by using an etching resist, etc. In this method, when the thickness of copper plating is thick or approximately 18 μm, a problem is that an obtained pattern form is similar to the above forms. When the thickness is thin or approximately 10 μm, the thickness of plating attached in a hole portion is insufficient so that reliability is poor. Further, there is also a problem with regard to adhesive strength to copper.
Further, when copper plating is carried out by a fully additive method, a problem is that copper foil adhesive strength is low even if the thickness of a copper layer is thick. On the other hand, there are a method in which an ultrathin copper foil is used, a plating layer is formed thereon by electroless copper plating and then a pattern is formed by a pattern copper plating method, and a method in which a thin electroless copper layer is formed on a substrate by a semi additive method and a pattern is formed by using the electroless copper layer by a pattern copper plating method. In these methods, however, the electroless copper layer is etched at its sides by a final flash etching to cause undercuts so that the above methods have a problem in adhesive strength to copper.